There are 2 8 ohm speakers in series with each other and there is a 16 ohm sub in parallel with the other two. A low pass filter is in series with the subwoofer. Now when the low pass blocks the signal the overall circuit impedance raises to 16 oms? and when the low pass filter passes the signal, the overall circuit impedence drops to 8 ohms?
To me, it looks like 8 ohms all the time. If you want to physically verify this, it can be done with a volt meter. Put a 200 ohm 25-watt resistor in series with the entire string. This will force the amp into a constant-voltage output, and won't hurt the amp at all (SS). Not the thing to do with tube amps. Substitute an 8 ohm resistor for the speaker cabinets. Drive it with enough power to get 0.8 volts or so. This is your calibration, and you can read "ohms" equivalent on the voltage scale. Remove the 8 ohm resistor and put the cabinet group back into place. Put the volt meter across the cabinet load and measure again. It will fluctuate with frequency, and tuning (vented boxes).
